The amount of information generated in society is growing exponentially. Moreover, the data is made available in more than one dimension across different media, such as video, audio, and text. This mass of multimedia information poses serious technological challenges in terms of how multimedia data can be integrated, processed, organized, and indexed in a semantically meaningful manner to facilitate effective retrieval.
When the amount of data is small, a user can retrieve desired content in a linear fashion by simply browsing the data sequentially. With the large amounts of data now available, and expected to grow in the future, such linear searching is not longer feasible. One example used daily is a table of contents for a book. The larger the amount of information, the more the abstraction needed to create the table of contents. For instance, while dividing an article into a few sections may suffice, a book may need subsection or even sub-subsections for lower level details and chapters for higher level abstraction. Furthermore, when the number of books published grows rapidly, in order to assist people to choose appropriate books to buy, books are grouped into different categories such as physics, mathematics, and computer hardware or into even higher levels of abstraction such as categories of literature, science, travel, or cooking.
Usually, a content structure is designed by the producer before the data is being generated and recorded. To enable future content based retrieval, such intended semantic structure (metadata) should be conveyed simultaneously to the users as the content (data) is delivered. In this way, users can choose what they desire based on the description in such metadata. For example, every book or magazine is published together with its table of contents, through which users can find the page number (index) where the desired information is printed by simply jumping to the page.
There are different methods to generate the above described abstraction or metadata. The most intuitive one is to do it manually as in the case of books (table of contents) or broadcast news (closed caption) delivered from major American national broadcast news companies. Since manual generation of index is very labor intensive, and thus, expensive, most types of digital data in practice is still delivered without metadata attached.